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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

236

SPECIAL BRONZES

bells, very hard and sonorous but brittle, contain usually 75-80% Cu, 20-25%
Sn, and sometimes small quantities of zinc, lead, silver, etc. Silver-coloured
bronzes for small, highly sonorous bells contain 40-42% Cu and 58-60% Sn.
Statuary bronze contains also considerable proportions of zinc (78-90% Cu
2-4% Sn, 10-18% Zn).

As a rule commercial bronzes, besides copper and tin, contain small quan-
tities of other elements (especially zinc, iron, manganese, nickel, lead, antimony,
phosphorus, arsenic, sulphur, etc.), which, even in small proportions, may pro-
foundly modify the properties of the alloy (see Special Bronzes) ; so much is this
the case that it is not always easy to decide if these elements are impurities of
the raw materials used or if they have been added to obtain certain, specific
properties.

Zinc in small quantities (0-5-2%) renders bronze more easy to cast and
increases its strength and elasticity ; if, however, the proportion is slightly
greater than 2%, the bronze loses its hardness and strength and assumes pro-
perties approximating to those of the brasses.

The presence of lead is especially harmful in bronzes for machines and tools
even if it is only slightly greater than 0-5%, since it diminishes the homogeneity
of the alloy. In proportions not exceeding 1-5%, iron exerts an action analogous
to that of zinc and increases considerably the hardness of the bronze. Arsenic
antimony, bismuth and sulphur are highly injurious, and even in the proportion
of 0-1% render the bronze brittle. Nickel, manganese and phosphorus improve
the quality of the alloy and are often added (see Nickel-bronze, Manganese-
bronze, Phosphor-bronze, etc.).

SPECIAL   BRONZES

These are copper-tin alloys containing another element added with a
definite object. Those in more common use are : phosphor-, silicon-,
manganese-, lead-, nickel-, lead-nickel- and aluminium-bronzes.

Phosphor-bronzes

Phosphor-bronzes are so called because during their preparation a small
quantity of phosphorus is added as phosphor-copper or phosphor-tin with
a view to reduce the metallic oxides contained in the metal and, therefore,
to impart greater hardness, strength, elasticity, etc. The attempt is usually
made to add the quantity of phosphorus just necessary for the deoxidation
(there may, therefore, be. phosphor-bronzes free from phosphorus), or at
most only a slight excess, so that only a very small amount passes into the
alloy. The analysis includes determinations of the usual constituents
and impurities and also that of phosphorus.

1. Determination of the Phosphorus.(a) IN ABSENCE OF ARSENIC.
2 grams of the sample are dissolved in aqua regia or in hydrochloric
acid and potassium chlorate, the solution being repeatedly evaporated to
dryness with hydrochloric acid, the residue then dissolved in dilute hydro-
chloric acid and water and the phosphoric acid precipitated with ammonium
molybdate as in the determination of phosphorus in iron (see p. 173).

(6) IK PRESENCE OF ARSENIC. After the repeated evaporation with
hydrochloric acid, the residue is taken up twice in hydrobrornic acid and
evaporated to dryness each time. The residue is then taken up in cone,
hydrochloric acid, evaporated to dryness, the residue dissolved in dilutereadwell:   Analytical Chemistry, Vol. II.to flow immediately the elec-